Because when external table is declared, default table path is changed to specified location in hive metadata which contains in metastore, but about partition, nothing is changed, so, we must manually add those metadata.

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ALTERTABLEuserADDPARTITION(date='2010-02-22');

Every time a new data=... folder (partition) is created, we must manually alter the table to add partition information.